1. Field of the Invention
The present invention relates to a part-fastening structure for: fastening a tubular first part, fitted to an outer periphery of a rotary shaft, to the rotary shaft in an axial direction with a first nut member which is screwed onto the outer periphery of the rotary shaft; and fastening a second part, fitted to an outer periphery of the first part, to the first part in the axial direction with a second nut member which is screwed onto the outer periphery of the first part.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2006-189065 has made publicly known a part-fastening structure which is used to fasten a part, fitted to an outer periphery of a rotary shaft, to the rotary shaft in an axial direction with nuts. In the part-fastening structure, a first external thread portion with a larger diameter and a second external thread portion with a smaller diameter, which are different in twisting direction from each other, are continuously formed on the outer periphery of the rotary shaft. The part is fastened to the rotary shaft with a first nut which is screwed onto the first external thread portion. Thereafter, a second nut screwed onto the second external thread portion is brought into pressure contact with the first nut. Thereby, the first nut and the second nut are prevented from becoming loosened.
The foregoing part-fastening structure can prevent the first and second nuts from becoming loosened, when: sufficiently large frictional force are acting on contact portions of the respectively first and second nuts; and the two nuts accordingly do not turn relative to each other. On the other hand, insufficient frictional force raises not only a problem that the two nuts are likely to become loosened because the contact portions of the two respective nuts slip over each other, but also a problem that a dimension of the rotary shaft in the axial direction increases by the second nut needed to prevent loosening.